The invention relates to a method for operating a power plant with a closed or quasi closed CO2 process, wherein the power plant includes at least one compressor, at least one turbine, at least one generator, at least one recuperator and at least one burner unit with at least one oxygen separation device and at least one burner, wherein the burner unit is supplied with oxygen-containing gas, compressed and heated CO2 medium from the CO2 process, and fuel or a fuel mixture. The invention also relates to a power plant having at least the above-described features.
A conventional power plant as well as a method for operating such a power plant is disclosed in WO 98/55208. The disclosed power plant includes a turbine in its CO2 cycle that drives a compressor and a generator via a common shaft. Also provided are a recuperator or heat exchanger as well as a burner unit, whereby this burner unit in the disclosed power plant comprises a first burner, an oxygen separation device, and a second burner. Oxygen-containing gas, for example, compressed ambient air, a compressed, heated CO2 medium of the CO2 process, and fuel are fed to the burner unit. The oxygen separation device contains oxygen separation means that remove oxygen from the oxygen-containing gas and feed it to the CO2 medium. The oxygen separation device, therefore, on the one hand, reduces the oxygen content of the oxygen-containing gas, and on the other hand enriches the CO2 medium with oxygen. The oxygen-enriched CO2 medium is burned in the second burner following the oxygen separation device together with the fuel, producing hot CO2 medium. This hot CO2 medium is fed to the turbine and expanded there. The expanded CO2 medium is then fed to the recuperator, where it cools down. Then the expanded and cooled CO2 medium is fed to the compressor and is then heated in the recuperator. The heated and compressed CO2 medium is then again fed to the inlet side of the oxygen separation device. This results in a closed or quasi closed CO2 process, from which the damaging CO2 can be removed with relatively little expenditure and without risk to the environment. Accordingly, the CO2 emissions associated with the burning of fossil fuels can be significantly reduced in such a power plant.
The core idea of a power plant with CO2 process is that pure oxygen is added as an oxidant to the CO2 medium. The combustion process with molecular oxygen produces a waste gas consisting essentially only of CO2 and H2O, which significantly simplifies after treatment of this CO2 medium, for example, in order to remove the H2O or CO2.
Since oxygen, which is produced in refrigerated plants, is very expensive, new technologies have been developed for producing oxygen. In this context, oxygen separation devices that are provided with a membrane that conducts oxygen ions and electrons, so-called MCM membranes (mixed conducting membranes), play an important role. Such an MCM membrane is provided with a retention side, on which the oxygen-containing gas is located, and a pass-through side, on which the gas to be enriched, for example CO2, is located. The MCM membrane transports oxygen ions from the retention side to the pass-through side and causes an electron transport from the pass-through side to the retention side. This causes oxygen to be removed from the gas on the retention side and to be fed to the gas on the passthrough side. In order to increase the efficiency of such an MCM membrane, it is advantageous to set a relatively high flow speed on the pass-through side so that the oxygen concentration on the pass-through side is as low as possible. It is advantageous for a long useful life of the MCM membrane to perform the following process steps independently from each other in separate units: Heating of the oxygen-containing gas, heating of the CO2 medium, transport of the oxygen from the oxygen-containing gas to the CO2 medium, and combustion of the oxygen-enriched CO2 medium with fuel. The functional separation of these procedures makes it possible to optimize the individual process steps separately, whereby, in particular, the useful life of the MCM membrane can be increased. It is particularly important for the efficiency of the system that the oxygen separation device or its MCM membrane has an optimum operating temperature that is relatively high. To enable the MCM membrane to reach its high operating temperature, the CO2 medium is preheated in the first burner preceding the oxygen separation device in a conventional power plant. The oxygen-containing gas is also preheated in a third burner preceding the oxygen separation device.
WO 98/55394 discloses a power plant in which the oxygen separation device is constructed as a so-called membrane reactor in which the enrichment of the CO2 medium with oxygen and the combustion with the fuel take place more or less simultaneously. Such a membrane reactor essentially corresponds to an oxygen separation device with MCM membrane, which is, however, operated at substantially higher temperatures.
EP 0 953 748 A1 discloses a power plant in which the CO2 medium is enriched with oxygen that is not produced with an oxygen separation device working with an MCM membrane, but with a cryotechnology plant.
Another power plant with CO2 process in which the oxygen enrichment takes place with oxygen produced with cryotechnology is known from EP 0 939 199 A1.
U.S. Pat. No. 5,976,223 discloses a device for producing carbon dioxide and oxygen that works with two oxygen separation devices that each are equipped with an MCM membrane. The first oxygen separation device, which functions as a membrane reactor, is supplied with oxygen-containing gas that has been compressed and heated on the retention side. On the retention side, a gaseous fuel is supplied that reacts with the supplied oxygen and forms carbon dioxide. The oxygen-containing gas with reduced oxygen content is heated by the exothermic reaction that takes place during this process. The oxygen-containing gas heated in this manner is then fed to the second oxygen separation device on its retention side. The desired oxygen then accumulates on the pass-through side of this second oxygen separation device.
Other methods and devices provided with oxygen separation devices working with MCM membranes are known, for example, from EP 0 882 486 A1 and U.S. Pat. No. 5,865,878.
The present invention provides a power plant and a method of operating a power plant in which an optimum operating temperature for an oxygen separation device has been improved.
According to an embodiment of the invention, a portion of the CO2 medium enriched with oxygen is burned in an additional burner following the oxygen separation device, and the oxygen enriched CO2 medium is mixed with the CO2 medium that has not yet been enriched with oxygen before it reaches the oxygen separation device. This measure makes it possible to increase the temperature of the CO2 medium fed into the oxygen separation device to such an extent that an optimum operating temperature can be set or adjusted for the oxygen separation device. In particular, this makes it possible to compensate for heat losses. By optimizing the operating temperature of the oxygen separation device, its efficiency and therefore also the efficiency of the power plant, is improved.
In another aspect of an embodiment of the invention, the burner unit may be provided with a heat exchanger, through which again on one side the CO2 medium flows upstream from the oxygen separation device, and on the other side the CO2 medium flows downstream from the oxygen separation device, whereby hot CO2 medium produced by the combustion in the additional burner is mixed downstream from the heat exchanger with the CO2 medium heated in the heat exchanger. The CO2 medium fed into the oxygen separation device is therefore heated primarily in the heat exchanger, whereby little external energy is required. The additional burner also compensates substantially for line losses and losses in the heat exchanger.
In an embodiment of the invention, the burner unit may be provided with at least one fuel cell that is supplied on the inlet side with the oxygen-enriched CO2 medium as well as with fuel, whereby a reaction takes place in the fuel cell that removes oxygen from the CO2 medium, heats the CO2 medium, and generates electrical power. The overall efficiency of the power plant can be improved by integrating one or more fuel cells into the burner unit. It is preferable if the fuel cell is arranged with respect to the supply of the burner with CO2 medium and fuel upstream from the burner, since this increases the process temperature in the burner and therefore also the temperature of the waste gases (CO2 medium) produced in the burner. A higher temperature in the CO2 process also increases the efficiency of the power plant.
If the oxygen-enriched CO2 medium is cooled in a heat exchanger, it is useful to branch off the portion of the CO2 medium necessary for the additional burner upstream from this heat exchanger, since this ensures that the heat exchanger then has a larger volume flow available and its efficiency is improved.
In accordance with an aspect of the invention, the flow temperature of the CO2 medium to be enriched with oxygen can be increased prior to its entrance into the oxygen separation device by using an additional burner. Such an additional burner can be controlled and regulated especially easily according to demand, ensuring the setting and maintaining of an optimum operating temperature for the oxygen separation device. This is especially advantageous for oxygen separation devices that work with an MCM membrane.